Winglets on Rudders

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DanBerger
Professional
Posts: 280
Joined: May 3rd, 2004, 3:29 pm
Boat Make/Model: SC 15 w/ spin!, SC 19
Location: Norfolk, VA

Winglets on Rudders

Post by DanBerger »

OK, Bill,

Why aren't we installing winglets on our rudders like the A cats? I understand a lot of the A cat success has to do with curved boards and pitch but the Moths were using them a long time ago before they were foiling.

I realize the boat would be a mother to launch from the beach and would put a lot of strain on the winglets when the rudders are up, but once you get the rudders down, it should be a huge increase in stability.

I ask this because I watched an A cat with winglets skip across the top of the waves while I hobby horsed through the waves and buried my bows in the back of waves.

Let's have it.
Bill Roberts
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Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Winglets on Rudders

Post by Bill Roberts »

Dan, SCs and ARCs are "one design" beach cats. They were designed back in the 70s and early 80s. Rudder foils for stability are a development thing that came along 20+ years later. Foil length and width and area and angle of attack has to be optimised to the boat it is used on.
A cats are a one person boat 18ft long. Length is a large provider of pitching stability.
If your boat was 18ft long or longer, it would pitch much less also.
First time riders on an ARC22 or 27 are often heard to say with only a minute or two of sailing on the boat, "Oh, this boat is so smooth". "What a nice ride".
DanBerger
Professional
Posts: 280
Joined: May 3rd, 2004, 3:29 pm
Boat Make/Model: SC 15 w/ spin!, SC 19
Location: Norfolk, VA

Re: Winglets on Rudders

Post by DanBerger »

I get the 'Nice ride' comment on my 15 all the time! I do understand hull length, but bolting the frames directly to the hulls has a lot to do with stability and no flex. The big hulls are pretty sweet, too--my friend says I'm sailing on a fiberglass cloud. He's a hater, though.

You have done a lot of experimenting in the past 20 years, so I was wondering why you hadn't looked into winglets and possible after-market winglet kits. Didn't you come up with moving the daggers forward of the beam on the 21s? I'm sure your research led to the self-tackers and lower jibs??

I (and most people on this forum) look to you and Tom for technological advances for our boats. I certainly can't spend the time to do the research and testing, but I at least listen to what you two say and go from there.

I hear tom is making Carbon Fiber rudders now. Seems like it would be a good time to do a little experimenting with them, eh? Just sayin'
Bill Roberts
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Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Winglets on Rudders

Post by Bill Roberts »

Hi Dan,
If I knew how to attact a photo on this forum, I could send you pictures of pitch foils on RC30 rudders. Don't forget pitch foils have area and area makes drag 100% of the time. I am in contact with A cat sailors who finish within the top few in World Championship races. The cut down foredecks are found to pitchpole too easily and they are coming back up in height. Marstrom never cut the foredeck down on his boats. Mast sections are becoming more bendy. This isn't new technology. This is going around the barn agan for the umteenth time. Old lessons relearned.
As far as carbon rudders go: Tom and I have been building carbon boards and rudders for the 27s and 30s for about 35 years now. Carbon is used so that these parts on these boats can carry the loads without breaking. A carbon part on your boat would be a no benefit to the performance of your boat. The water would not know the difference but your wallet would.
Bill Roberts
Expert
Posts: 515
Joined: November 17th, 2003, 9:13 pm
Location: Stuart, Florida

Re: Winglets on Rudders

Post by Bill Roberts »

DAN, Relative to your comment about shared lift. The first SC to have "shared lift" was the SC17. The deep V bow on the 17 carries half of the side force from the sails and the oversize rudder carries the other half. The induced drag due to generating underwater side force equal and opposite to the side force from the sail is significantly reduced with this scheme. Some/most boardless beach cats use 100% of the hull length for making this lift. This is a lousey underwater aspect ratio, depth to length ratio, like one foot deep by 16feet long. Aspect ratio equals 1/16 = 0.0625. In the equation for induced drag, aspect ratio is in the denominator so as you can see it is a big drag driver. On the SC17 and 15 only the front 5 or so feet has a deep V hull shape that can generate side force. Due to the location of this deep V hull section relative to the CE in the sail plan, it carries only half of the sail side force. Aspect ratio = 1/5. The other 50% of the sail side force falls on the balanced rudder. Its aspect ratio is 8 ins wide by 32 ins deep for an aspect ratio of 32/8 = 4.0. The average effective aspect ratio for the SC17 and 15 is (0.2 + 4.0)/2 = 2.1. As you can see the hull induced drag for the SC boardless products is much less that the Hobie or Prindle or any other boardless beach cat. This is why your SC15 is faster that a H16 and is much much safer.....pitchpole resistance.
So, "shared lift" has been a part of SC since the beginning, 1970s.
One other point worth making. The round hull bottom shape of the SC17 and 15 at the aft end cannot generate side force. This part of the hull just slips sideways with side force. This lets the boat trun and tack easily. A hull shape that carries a hard corner underwater right into the transom and rudder does not want to slip sideways easily. In tacking the rudder asks the aft end of the hull to move sideways and the H and P hulls say "NO and I'm gonna make alot of drag when you make me move sideways also". This makes H and P hulls hard and slow to tack.
Dan, Aren't you glad you have a SC15! Sared Lift is 1970s technology and used again on the SC21 in the 21st century.
I think shared lift might be an excellent design concept for an Around the World Race catamaran. If reaching and running is most of the race, shared lift like the SC17 and 15 could be a winning way to go.
One other point about new catamaran products. I saw a Hobie Getaway the other day and the sailor was having a really hard time bringing the boat about. Later I saw the boat on a trailer and I noticed a "skeg" on the keel at the aft end of the boat about 2 ft long. No wonder the boat won't tack. You put a skeg along the keel at the aft end of a boat to make the boat, like a rowing shell for example, go in a straight line. The skeg resists sideways motion and makes the rowing shell go straight. Why would you put a skeg right in front of a rudder. Now when the rudder generates side force to make the boat turn/tack, the skeg says "NO" and makes alot of additional drag while tacking. This additional drag brings the boat to a stop at mid tack if it can even get that far through a tack. Now back the jib and if you don'r reverse the helm while backing up, the boat will return to the original tack/course. Boy, that Getaway is a fine design. I'm sure it is a good moneymaker; junk design but a good moneymaker.
DanBerger
Professional
Posts: 280
Joined: May 3rd, 2004, 3:29 pm
Boat Make/Model: SC 15 w/ spin!, SC 19
Location: Norfolk, VA

Re: Winglets on Rudders

Post by DanBerger »

Thanks for the headache, as usual Bill! It is nice to know why my boat acts like I know it does. Science and math. Awesome.

And, yes, I really appreciate my 15. I have never come close to pitch poling except in really heavy chop when I outrun the waves and bury the bows in the back of a big wave. In the flat stuff, I can never drive it too hard. You are right about the tacking, too, but I also feel that getting the bows out of the water by staying on the back of the boat helps it around, too. While that buries the sterns, they still slip around the tack. I usually start my tack from the wire and I sit in the windward back corner which lets the boat rotate (or pivot) around me, too.

Hobie has done really well with their Getaways and Waves. Those skegs help a lot, but they serve their purpose well. the Nacra 450, 5.0 and 5.7 have the same skegs but in fiberglass. I have sailed a 450 a lot and you're right about the tacking--not easy. However, they do have that tall thin bow section that when driving really lifts the boat. The getaways are party barges and I think the deciding factor in building them was that they were mostly rentals and needed to be bullet proof. Round bottoms or 'V" bottoms like the 16 wouldn't work so a nice, beefy skeg would give it something to sit on. That's at least the way I see it

I have sailed a Getaway and they are fun boats with wings, built in cup holders and coolers. They aren't performance boats by far, but comfortable. They are HEAVY!! We do a distance race in the Outer Banks and they let the Getaways race. Their handicap is so high that we finish the race and an hour later, all the Getaways show up and win the race! The worst part is that the sailors are completely wasted, too! They are pretty fast down wind, but can't point for Sh!t!
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